Switches connect two or more separate LAN's. The switch has ports for LAN interfaces which connect individually to separate LAN's. A packet which is present on a first LAN, is read by the switch, and the switch determines from the destination address of the packet, whether the packet should be transmitted to a second LAN. The switch reads the destination address, and through the use of a look-up table in the memory, determines if the destination address is in the first or second LAN. If the destination address is in the second LAN, the packet is forwarded to the port or interface for the second LAN. If the destination address is in the first LAN, the switch does not forward the packet to the second LAN. If the look-up table in the memory does not have an entry for that particular destination address, the switch can either ignore the packet or forward the packet to all its ports or LAN interfaces. Likewise a packet circulating in the second LAN is read by the second LAN interface means, and the destination address of this packet is compared with the look-up table in the memory to determine if the destination address of the packet from the second LAN is located in the first LAN. A decision as to what to do with a packet having a destination address not listed in the look-up table, depends on the configuration of the switch.
As networks grow larger and larger, and the workstations connected to the network change, the address look-up table in the switch must correspondingly change. It is possible to change the address look-up table manually each time a change is made to one of the networks connected to the switch. However, this can be time consuming and require much effort on behalf of the manager of the computer network.
It is also possible to have the switch automatically update the look-up table. A processor in the look-up table reads the source address of all packets received on each LAN interface. The processor then knows that the LAN of the respective interface contains the workstations having the source address of the packets received on the respective interface. The processor then uses this information to update the address look-up tables. In this way, as soon as a workstation sends a packet onto the first LAN, and the processor in the switch reads the packet, the address look-up table in the switch can be updated to indicate that the particular workstation resides on the first LAN. This allows the look-up table to be updated automatically.
Whether or not the look-up table is updated automatically, or manually, the memory which stores the look-up table needs to be accessed by all of the LAN interfaces, and the processor which performs the updating. Depending on the type of memory, only one device is able to access the memory at one time, or only one device is able to access a particular memory location at a time.